Title page for ETD etd-04032007-084834

The Effects of Trawling and Habitat Use on Red Snapper and the Associated Community

Degree

Doctor of Philosophy (Ph.D.)

Department

Oceanography & Coastal Sciences

Advisory Committee

Advisor Name

Title

James H. Cowan, Jr.

Committee Chair

Charles A. Wilson

Committee Member

James P. Geaghan

Committee Member

Lawrence J. Rouse, Jr.

Committee Member

William F. Patterson, III

Committee Member

Keywords

red snapper

trawling

habitat

life history

community

Date of Defense

2007-03-23

Availability

unrestricted

Abstract

The primary impediment to the recovery of Gulf of Mexico (GOM) red snapper is believed to be high levels of bycatch of age 0 and age 1 individuals in shrimp trawls. Thus, conservation of GOM red snapper involves evaluating both habitat-specific function and effects of shrimp trawls on red snapper and the associated benthic ecosystem. The two goals of this study were to evaluate the effects commercial shrimp trawls have on juvenile red snapper life history parameters, on associated fish and invertebrate communities and their habitat, and to identify essential fish habitat (EFH) for red snapper by applying all four data quality levels specified by the National Marine Fisheries Service. Sampling with a suite of gear types occurred during quarterly cruises over a two-year period on the northern Gulf of Mexico continental shelf. Presence of commercial shrimp trawls had negative impacts on the benthic ecosystem at large. Red snapper were found to have truncated size distributions, slower growth rates in the fall, higher mortality rates, lower production potential, and different feeding dynamics, as revealed by differences in ä15N and ä34S, over habitats exposed to trawling. Differences in biotic communities, diversity indices, size structure, and habitat characteristics were also found between similar habitats in trawled versus non-trawled areas. Ontogenetic habitat shifts from low to higher relief habitats with increasing size and age were found for red snapper. Shell-rubble and reef habitats may be important for red snapper by providing protection from predators; however, the sand and mud habitat appears to be the most important for enhancing production and early life survival of age 0 fish based upon faster daily growth rates and higher production potential. Results of this study indicate the presence of commercial shrimp trawling negatively impacts the benthic ecosystem and post-settlement processes acting on age 0 red snapper. All habitats sampled in this study are important during some portion of the life history of red snapper. Thus, if shrimp closures are proposed in the GOM to enhance survival of age 0 and age 1 red snapper, then all habitat types sampled in this study will need to be protected.